We propose to study the effects of calcium on renal tubule epithelial transport using the technic of in vitro microperfusion, and the specific aims may be divided into two categories. Firstly, the modification of fluid and electrolyte transport processes by calcium will be studied in individual nephron segments. To determine the effect of altering ambient calcium, the calcium concentration will be varied in the perfusing or bathing solution or calcium will be replaced in these solutions with magnesium or manganese. Phosphate transport will be examined in proximal tubule segments from animals in which chronic hypercalcemia or hypocalcemia has been induced. Also to be studied is the effect of increasing cytosolic calcium with ionophores, and inhibitors of mitochondrial Ca-ATPase and the Na/Ca antiport at the basolateral membrane. The contribution of the Ca-calmodulin complex to transport processes will be examined by the use of its inhbitors. The studies in the second category will examine the role of changes in cytosolic calcium by various hormones and the effect this has on tubular transport. To determine if calcium entry is an important mechanism, calcium-channel blockers and low external calcium will be used to attenuate this entry. The role of membrane-bound calcium and calcium-stimulated C-kinase activity will be examined by the addition of lanthanum and phorbol esters, respectively, to the perfusing and/or bathing solutions. Finally, the role of intracellular organelles which store calcium and the extrusion of calcium at the basolateral membrane will be studied. Inhibitors of mitochondrial Ca-ATPase, endoplasmic reticulum calcium release, and the Na/Ca antiport will be used to study this aspect. These studies should provide a better understanding of the role of calcium on various transport processes in the mammalian nephron.